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| Home > Publications database > Effect of the fractionation and immobilization on the sorption properties of humic acid |
| Dissertation / PhD Thesis/Book | PreJuSER-33181 |
2003
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/147
Report No.: Juel-4046
Abstract: Humic substances modify the surface of inorganic soil constituents changing the nature and amount of adsorption sites for contaminants and also influencing the particle-particle interactions and thus the mobility and transport behavior of the soil particles. The aim of this PhD work was to investigate the effects of the interactions between two important soil components, aluminum oxide and humic acid, on the sorption behavior of 2,4-dichlorophenol under laboratory conditions selected to model the soil systems. Humic acid is very heterogeneous in terms of physical and chemical properties. To reduce the chemical heterogeneity of the extracted humic acid (from the A$_{p}$ horizon of the Orthic Luvisol, Merzenhausen, Germany), a fractionation scheme using the ultrafiltration technique was used to obtain eight size fractions of the humic acid. The extracted humic acid and its fractions were characterized by potentiometric acid-base titration, elemental analysis and different spectroscopic methods (NMR, UV-VIS and FT-IR spectroscopy). Clear chemical differences between the humic acid size fractions were observed. Smaller size fractions of the soil humic acid contained more chargeable functional groups and larger percentage of aromatic carbon than the larger size fractions. Conversely, the percentage of aliphatic carbon increased with increasing apparent molecular weight. Moreover, the solid-state $^{19}$F-NMR was used to study the sorptive uptake of hexafluorobenzene by the humic acid and its fractions. It was found that humic acid molecules have different chemical environments into which organic pollutant such as hexafluorobenzene can sorb. Small humic acid molecules have at least three sorption sites (“rigid”, “soft” and other new domains) that are more clearly defined and homogeneous than the sorption domains found in larger humic acid molecules. The effect of the pH and the electrolyte concentration on the adsorption of the humic acid onto alumina surfaces as well as on the colloidal stability of these systems were studied. To better understand the binding mechanisms these results were also compared to those of polyacrylic acid. The adsorption of humic acid or polyacrylic acid to alumina varied with pH and electrolyte concentration, suggesting that the conformation of the humic acid or polyacrylic acid in solution significantly determines their structures on the mineral surface. At low pH (< point of zero charge (PZC) of alumina), increasing amounts of humic acid or polyacrylic acid are adsorbed on the alumina surface with increasing concentrations of solutes, resulting in a charge reversal from positive to negative net total particle charge whilst at high pH (> PZC), the electrophoretic mobility was shifted to more negative values. The colloidal stability of the alumina dispersions containing increasing amounts of the added humic acid and polyacrylic acid, respectively, was monitored using the dynamic light scattering technique. The maximum aggregate size was observed around the zero electrophoretic mobility, indicating the importance of the charge neutralization mechanism. By comparing the adsorption of the humic acid fractions on alumina surfaces, it was found that the adsorbed amount increases with increasing humic acid molecular size. Furthermore, an increase in the rise of the adsorption isotherm in the plateau regions by increasing the humic acid fraction size was also observed which indicates a higher contribution of the hydrophobic interactions due to the increase in the aliphatic carbon and the decrease in the chargeable groups. The sorption results of 2,4-dichlorophenol on the immobilized humic acid at different pH and salt concentrations revealed that the value of the sorption coefficient decreases as the ionic strength decreases or the pH value increases which gives direct evidence for the importance of the conformation of the immobilized HA. These sorption isotherms can be described as a linear isotherm, which indicates that the sorption of 2,4-dichlorophenol is predominantly a partitioning process between the aqueous phase and the immobilized humic acid.
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